Process for producing sheets of magnetic materials



Sept. 8, 1964 D, s. SHULL, JR

PROCESS FOR PRODUCING SHEETS OF MAGNETIC MATERIALS Filed Nov. 17, 1960 United States Patent O 3,148,092 PROCESS FOR PRDUClNG SHEETS F MAGNETC MATERIALS Daniel S. Shull, Jr., Forest Hills, Pa., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a 'cor eration of Pennsylvania p Filed Nov. 17, 1960, Ser. No. 69,939

3 Claims. (Cl. 148-103) This invention relates to a process for producing sheets of cobalt-iron alloys having improved magnetic properties.

Cobalt-iron alloy magnetic material having a relatively high cobalt content has outstanding properties which make the alloys attractive for many applications md particularly for aircraft and military applications. These desirable properties are: (1) a high saturation induction, which makes possible drastic reductions in the size and weight of magnetic cores, (2) rectangular hysteresis loops when the alloy is suitable processed and annealed, which make the alloys applicable in saturable core devices, and (3) high Curie temperatures, which make them useful in elevated temperature environments.

However, the high cobalt alloys now in use have impoi-tant disadvantages. For example, a high cobalt alloy tends to assume a highly ordered crystalline lattice structure, which degrades the physical properties of the alloy to some extent. In avoiding such ordered structure arising in the alloys, rather complicated annealing cycles have had to be developed. Since the cost of the cobalt coniponent is high, the high cobalt content alloys having 50'% or more of cobalt are so expensive that they have areiatively limited field of application, even though with higher cobalt content in known compositions better magnetic properties are present. t

Accordingly, it is the object of this invention to provide a relatively simple process comprising vacuum melting, a selected rolling and working schedule and magnetic annealing for producting cobalt-iron alloy sheet having good magnetic properties and requiring substantially less than 50% by weight of cobalt.

A further object of this invention is the provision of a cobalt-iron alloy sheet having an extremely low content of detrimental impurities, hysteresis loops having a high degree of rectangularity, and a low 400 cycle core loss at high flux density, when the sheet is Wound in the form of a toroidal core.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

For a better understanding of the nature and objects of the invention, reference may be had to the following detailed description and to the drawing, in which:

The gure is a graph plotting ilux density against core loss for several alloys produced in accordance with this invention and for an alloy made in accordance with the prior art.

Brieliy, this invention is directed to a process for making by closely controlled melting and heat treating techniques including annealing with cooling in a magnetic field, sheets of high purity cobalt-iron alloys having substantially less than 50% cobalt therein, and having good magnetic properties. The cobalt-iron alloy sheet made in accordance with this invention is particularly useful in thickness of from 0.002 inch to 0.050 inch.

The alloys of this invention are composed of from 25% to 45% by weight of cobalt, up to about 5% by weight of at least one element selected from the group consisting of vanadium and chromium, and the balance essentially iron except for small amounts of inadvertent impurities.

Fatented Sept. 8, 1964 A most favorable range for the vanadium and chromium additions is from 0.5% to 2%, by Weight. It will be understood that in the high purity alloys of this invention, small amounts of normally unavoidable impurities such as nickel, carbon, sulfur and phosphorus may be present. In general, nickel is not considered a detrimental impurity in the alloys of this invention and may be present in amounts of 0.2%, or even more With some beneit. On the other hand, carbon, sulfur and phosphorus are critically detrimental and the total amount of these elements should not exceed 0.02%. Traces of other impurities such as oxygen, nitrogen and the like may be present. The raw materials employed in making the alloy of this invention, that is, the cobalt, iron, vanadium or chromiuni must therefore be of high purity.

The process of the invention involves the steps of melting the several alloy constituents to form the alloy, subjecting the alloy to sub-atmospheric pressure While in the molten condition to reduce impurities, casting the alloy to ingot form, heating the ingot to a temperature in the range from 950 C. to 1200 C. in a protective atmosphere, hot rolling the ingot into strips of a thickness of from about 0.050 to 0.250 inch, reheating the strip thus formed to a temperature in the range from 850 C. to i000 C. for from l to 10 minutes, rapidly quenching the strip to produce a ductile structure and thereby facilitate cold Working, cold rolling the strip to eifect a reduction of at least 50%, and preferably from 80% to 98% reduction to produce a sheet of a predetermined thickness, finally annealing the sheet or a core made from it at a temperature of 850 C. to 950 C. for from about 10 minutes to four hours in an atmosphere of dry hydrogen having a dew point of 45 C. or less, then cooling the annealed sheet in a magnetic field of at least 1.5 oersteds at the rate of from 1 C./min. to 5 C./min. to a temperature in the range from 550 C. to 750 C., and then rapidly cooling the sheet in about 10 minutes to 200 C.

Within the broad range of cobalt content indicated above, it has been found that alloys containing about 27%, 35% and 43% cobalt, respectively, are particularly advantageous. While the binary alloys have been made successfully, such alloys are characterized by substantially poorer physical properties Which limits their usefulness. The ternary alloys containing vanadium or chromium, or both, therefore, have wider applicability because of superior physical properties.

The alloys of this invention have been found to exhibit the best magnetic properties when final annealing treatments are performed in the alpha region which, for these alloys, is at a temperature not in excess of 950 C. Also, while the ordering phenomena is not quite as marked in these alloys as in alloys having about 50% cobalt, and ordering becomes progressively less pronounced with decrease in cobalt contents, yet ordering will usually occur in these alloys when held for long periods of time at temperatures above about 300 or 350 C. and below the range of 550 C. to 750 C., the precise temperatures being dependent upon composition. Therefore, rapid cooling rates of at least 10 C./min. to 20 C./min. are employed when the sheets reach temperatures at 550 C. to 750 C. in order to cool them to below 30G-350 C. or room temperature in order to minimize ordering, and thus avoid the deleterious effects of this phenomenon on ductility.

The amount of reduction in the hot and cold working stages is not ciritical, the most important consideration during rolling being convenience in handling the material and arriving at the desired final thickness.

insofar as melting procedures are concerned, either 13 vacuum induction melting or vacuum arc melting may be employed.

In connection with the field strength for the magnetic anneal, the principal requirement is that the field strength exceed the coercive force of the material. A Value of 1.5 oersteds is satisfactory in these ailoys, but as much as 10 to 15 oersteds or even more, may be used with good results.

Illustrative of the practice of this invention, three ingots of the alloys were made at each of three nominal cobalt levels; 27%, 35% and 43%. At each cobalt level, one ingot was a binary cobalt-iron alloy, one ingot contained about 1% vanadium, and the third ingot contained about 1% chromium. No other additions were made to any of the ingots, and the analyzed composition of each ingot is given in Tabie I.

TABLE I Composition of Cobalt-Iron Ingols Ingot No. Co Fe C S P 27. 71.5 0.0016 0. 0042 0.0005 25. 9 71.1 0. 0007 0. 0038 0. 0005 26. 9 71. 0 0. 0009 0. 0033 0. 0005 35. 0 64.1 0. 0006 0. 0029 0.0005 33. 7 63. 4 0.0013 0. 0038 0.0005 34. 7 63. 0 0. 0011 0. 0033 0. 0005 43. 0 56. 9 0. 0020 0. 0005 0. 0005 42. i 5G. i 0. 0013 0.0007 0. 0005 42. 5 56. 2 0. 0014 0. 0009 0. 0007 Because of the importance of high purity in obtaining the desired properties in these magnetic materials the iron, cobalt, chromium and vanadium used were all of the highest purity electrolytic grades commercially available. The nickel is present as an impurity because it is most difficult to separate it from cobalt, with which it is always associated in the ores thereof.

All the ingots were vacuum induction melted in a magnesium oxide crucible, with a low partial pressure helium atmosphere providing a sub-atmospheric pressure environment. The charge Was placed in the crucible prior to heating, and the vacuum was maintained until just before the charge melted, at which time a small pressure of helium was introduced into the furnace space. The ingots were cast in a slab mold to eliminate the need for any forging operation prior to hot rolling. The hot top of each ingot was removed, leaving a slab suitable for further processing.

A portion of each slab was then hot rolled to a strip of a thickness of approximately 90 mils. 1n most cases, this was done without reheating in order to minimize grain growth. Most of the ingots were heated to a temperature of 1000 C. prior to hot rolling in either a protective argon or hydrogen atmosphere. For several of the ingots which tended to be brittle during the latter stages of hot rolling, a temperature of 1100 C. was ernployed.

After hot rolling, all strip was reheated to 900 C. and then quenched in ice brine to produce a more ductile structure. The reheating of the strips required from 2 to 5 minutes and was carried out in a hydrogen or argon atmosphere. After quenching, the strips were prepared for cold rolling by pickling in a solution of hydrochloric and nitric acids until clean. The strips were cold rolled to sheets of a thickness of four mils, and slit apart into one quarter inch and the remainder into one half inch wide tape. No intermediate anneals were used. All rolling was done without tension.

The four mill tape was deburred, degreased, and insulated With magnesium oxide. The tape was then wound into toroidal cores. The cores Were annealed in a dry hydrogen atmosphere having a dew point of 45 C. or lower. The anneal was carried out at a temperature between 850 C. and 950 C. some for ten minutes, and others for three hours. After annealing, the cores were cooled at a controlled rate of 2 C. per minute in a eld of 10 oersteds until a temperature in the range between 550 C. and 750 C. was reached. Then the cores were rapidly cooled to room temperature, at a rate to reach C. in 10 minutes or less. During the rapid cooling, the magnetic field of about ten oersteds was maintained.

The saturation induction, approaching 24,000 gauss in all of the alloy cores of this invention, compared with about 21,000 gauss for pure iron, is of great practical signiticance. It is this characteristic that permits a great reduction in size of magnetic devices. The magnetic properties of the cores were evaluated using both D C. and 400 c.p.s. alternating current to determine their core loss properties. The D.C. test results indicated that all cornpositions benefited by the magnetic anneal and the hysteresis loops obtained possess a high degree of rectangularity. It may be stated for the ternary alloys that the hysteresis loops obtained for toroidal cores have a rectangularity as measured by the ratio Br/Bm of at least about 70%. The ternary 43% cobalt alloys displayed the best magnetic properties: Bm (at 50 oersteds)=22.63 to 22.99 kilogauss, B,=16.08 to 17.16 kilogauss, and Hc=0.216 to 0.259 oe. Table II sets forth additional data on the D.C. magnetic properties of two cores of each of these alloys annealed as indicated.

TABLE II D.C. Magnetic Properties of Cobalt-Iron Alloy Cores Heat No. Time ot Anneal Bm at 50 Br, kg. H, oer.

oer. kg.

19. 96 15. 84 0. 308 19. 97 1G. 47 0. 573 19. 5G 14. 50 0. 571 18. 00 15.39 0. 663 19. 76 13. 60 0. 602 19. 19 16.02 0.619 21. 00 13. 51 O. 241 22.16 15. 60 0. 411 21. 99 14. 83 0. 350 22. 67 17. 08 0. 394 21. 25 15. 52 0.396 22. 00 16. 56 0. 378 10 min. G inch tape 22.27 14. 29 0.281

1" x it core.)

Note: The first core for each alloy, except 1741, in Table II was 7/8 inch outside diameter and EVt inch inside diameter, While the second was l inch outside diameter and 5%: inch inside diameter; all made from 1A inch tape.

In alloys treated in accordance with this invention it has been found that the D.C. coercive force will not exceed a maximum value of 0.7.

When these materials were annealed without a magnetic eld, much poorer D.C. properties were obtained. For example, the coercive force obtained in the 43 alloy increased from 0.216 to 0.958 oe., and residual induction dropped from 17.16 to 6.80 kilogauss. Correspondingly inferior results were obtained with the other compositions; thus, the magnetic anneal is essential whenever the best properties are desired. The difference in properties of cores held at the upper annealing temperature for ten minutes or for three hours was not large. In general, coercive force was lower with the three hour anneal, while residual induction was higher with the ten minute anneal.

The above DC. properties of high purity, magnetically annealed alloys, are considerably improved over those of similar commercial materials. Typical values of coercive force for air melted, non-magnetically annealed compositions are 2.05 oe. for a 27% cobalt-iron alloy and 1.80 oe. for a 35% cobalt-iron alloy. The commercial grade materials are not normally annealed in a magnetic field, since compositions containing substantial amounts of the usual deleterious impurities do not respond well to the iield anneal.

1t is also significant that the A C. properties of the tersignicantly improve the nary alloys were substantially improved, as indicated below in Table III:

TABLE III 400 c.p.s. Core Loss of Cobalt-Iron Alloy Cores Annealed for Minutes Core Loss (Watts/Pound) at- Heat N o.

10 kg. 12 kg. 14 kg. 16 kg. 17 kg. 18 kg.

Note: These cores were 1 inch outer diameter, 3/4 inch inner diameter, and of 1A inch wide tape, except 1741 which was of 1/2 inch wide tape.

From Table III it will be noted that the 400 c.p.s. core loss of all of the alloys is less than 25 watts per pound at a ilux density of 16 to 18 kilogauss, which is the iiux density range of particular interest. At the 27% cobalt level in the ternary alloys, the 400 c.p.s. core loss ranges from about 18 to 23 Watts per pound; all core loss values falling below 24 watts per pound. At the 35% cobalt level in the ternary alloys, all core loss values fell below 16 watts per pound. At the 43% cobalt level in the ternary alloys, all core loss values fall below 12.5 watts per pound.

The core loss properties at 400 c.p.s. for some of the improved alloys of this invention are shown in the figure of the drawing, with data on several commercial materials for comparison. The curves shown at each of the three cobalt levels are for the ternary compositions with 1% chromium. The improvement at the 27% cobalt level is seen to be quite substantial. The figure indicates that the vacuum melted, magnetically annealed 27% alloy, has losses about half as great as those of the air melted, nonmagnetically annealed commercial 27% alloy. Comparable improvement is obtained for the alloys of this invention at the 35% and the 43% cobalt levels. At llux densities above 17 kilogauss, the improved 35% and 43% cobalt-iron alloys have losses lower than commercial grade oriented silicon-iron.

The time at reheating temperature after hot working and prior to cold working of the hot rolled strip has been found to be critical for the following reasons. A disordered structure after quenching the rehe-ated strip is necessary in order to obtain good ductility. Thus, the material must be heated -to a temperature above the orderdisorder transformation, that is, to insure it is all in the disorder region, prior to quenching. By quenching rapidly from a temperature in the disorder region, a disordered structure is maintained at cold working temperatures. However, if the material is heated too long in this region the ductility is impaired even though a disordered structure is obtained after quenching. As indicated previously, reheating for from 1 to l0 minutes has been found to give good results and any excessive time should be avoided.

The alloy sheets of this invention can be readily duplicated. The method described makes it possible to produce sheets of the alloy and to fabricate them readily into the form necessary for utilizing the magnetic :alloy as a component of electrical apparatus. The alloy sheets of this invention can be applied to the magnetic core structures of transformers and rotating equipment and will performance of such structures. The high operating inductions possible with the use of these alloy sheets will significantly reduce the size and weight of inductive components. Size reductions of as much as 25% can be achieved, a feature of particular importance in the design of aircraft components. Alloy sheets made in accordance with this invention will have good high temperature properties, capable of functioning in environments Where 'the performance of most magnetic alloys will be seriously degraded.

Since certain obvious changes may be made in the processes described and different embodiments of the invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or taken in connection with the =ac companying drawings -shall be interpreted as illustrative and not in a limiting sense.

I claim as my invention:

l. In a process for making magnetic alloy sheet from an alloy comprising by Weight, from 25% to 45% cobalt, up to 5% of at least one element selected from the group consisting of vanadium and chromium, and the balance iron except for small amounts of impurities, fthe deleterious impurities amounting to no more than 0.02%, the steps comprising, melting the alloy, subjecting the alloy to subat-mospheric pressure while in the molten condition, casting an ingot of the alloy, heating the ingot in a protective atmosphere to a temperat-ure in the range from 950 C. lto l200 C., hot rolling the ingot in a protective atmosphere to form strip, reheatiug the strip to a temperature in the range from 850 C. to 1000 C. for from 1 to 10 minutes, rapidly quenching the strip to produce a ductile structure, cold rolling the strip to effect a reduction of at least 50% to a sheet of predetermined thickness, iinal annealing the sheet at a temperature of 850 C. to 950 C. for from about l0 minutes to four hours in an atmosphere of dry hydrogen having a `dew point of -45 C. or lower, cooling the sheet in a magnetic field of at least 1.5 oersteds at a rate of from 1 C. per minute to 5 C. per minute to a temperature of from 550 C. to 750 C., and then rapidly cooling the sheet, the sheet when tested in toroidal core form being characterized by a hysteresis curve of a high degree of rectangularity as indicated by a fir/t?m ratio of at least about and a 400 c.p.s. core loss of less than 25 watts per pound at 16 to 18 kilogauss.

2. In a process for making magnetic alloy sheet from an alloy comprising, by weight, about 27% cobalt, from 0.5% to 2% of at least one element selected from the group consisting of vanadium and chromium, and the balance iron except for small amounts of impurities, deleterious impurities being present in amounts not exceeding 0.02%, the steps comprising, melting the alloy in a sub-atmospheric pressure environment, casting an ingot of the alloy, heating the ingot in a protective atmosphere of argon to a temperature in the range from 950 C. to 1200 C., hot rolling the ingot in a protective atmosphere of argon to form a strip, reheating the strip to a temperature in the range from 850 C. to 1000 C. for from 2 to 5 minutes, rapidly quenching the strip in brine to produce a `ductile structure, pickling the strip, cold rolling the strip to effect a reduction of from to 98% and thereby produce a sheet of predetermined thickness, nal annealing the sheet at la temperature of 850 C. to 950 C. for from about l0 minutes to four hours in an atmospher-e of dry hydrogen having a dew point of -45 C. or lower, and cooling the sheet in a magnetic field of at least 1.5 oersteds at a rate of from 1 C. per minute to 5 C per minute to a temperature of from 550 C. to 750 C., and then rapidly cooling the sheet at a rate of at least 10 C. per minlute, the sheet when rtested in toroidal core form being characterized by a hysteresis curve of a high degree of rectangularity as indicated by a 19T/Bm ratio of at least about 70%, and a 400 c.p.s. core loss of less than 24 watts per pound at 16 to 18 kilogauss.

3. In a process for making magnetic alloy sheet from an alloy comprising, by weight, from 25% to 45% cobalt, up to about 2% of at least one element selected from the group consisting of vanadium and chromium, and the balance iron except for small amounts of impurities, with deleterious impurities not exceeding 0.02%, the steps comprising, melting the alloy `below atmospheric pressure, cooling the alloy to ingolt form, heating the ingot in a protective atmosphere of argon to a temperature of yabout 1000 C., hot rolling the ingot in .a protective atmosphere to form a strip, reheating the strip to a temperature of about 900 C. for from 2 to 5 minutes, rapidly quenching the strip in brine to produce a disordered ductile structure, pickling the strip, cold rolling the strip to a sheet of predetermined thickness, iinal annealing the sheet at a temperature of 850 C. to 950 C. for from 10 minutes to four hours in an atmosphere of dry hydrogen having a dew point of -45 C. or lower, and cooling the sheet in a magnetic eld of about 10 oersteds at a rate of from 1 C. per minute to 5 C. per minute until the temperature of the strip is in the range from 550 C. to 750 C., and then rapidly cooling the sheet, the sheet when tested in toroidal core form being characterized by a hysteresis curve of a high degree of rectangularity as indicated by a Br/Bm ratio of at least about 70%, and a 400 cps. core loss of less than 25 Watts per pound at 16 to 18 kilogauss.

References Cited in the le of this patent UNITED STATES PATENTS OTHER REFERENCES E. H. Williams: Saturation Value of the Intensity of Y Magnetization and the Theory of the Hysteresis Loop,

Physical Review, vol. 6, No. 5, 1915, pages 404-409.

Ferroinagnetism, R. M. Bozorth, 1951, published by D. Van llostrand Company, Inc., N.Y., pages 20-24,

n 35-38, 58-66 and 190-204.

Geisler et al.: Magnetic Anncaling of a Co-Fe Alloy Journal of Metals, vol. 5, June 1953, pp. S13-820. 

1. IN A PROCESS FOR MAKING MAGENTIC ALLOY SHEET FROM AN ALLOY COMPRISING BY WEIGHT, FROM 25% TO 45% COBALT, UP TO 5% OF AT LEAST ONE ELEMENT SELECTED FROM THE GROUP CONSISTING OF VANADIUM AND CHROMIUM, AND THE BALANCE IRON EXCEPT FOR SMALL AMOUNTS OF IMPURITIES, THE DELETERIOUS IMPURITIES AMOUNTING TO NO MORE THAN 0.02%, THE STEPS COMPRISING, MELTING THE ALLOY, SUBJECTING THE ALLOY TO SUBATMOSPHERIC PRESSURE WHILE IN THE MOLTEN CONDITION, CASTING AN INGOT OF THE ALLOY, HEATING THE INGOT IN A PROTECTIVE ATOMSPHERE TO A TEMPERTURE IN THE RANGE FROM 950*C. TO 1200*C., HOT ROLLING THE INGOT IN A PROTECTIVE ATMOSPHERE TO FORM STRIP, REHEATING THE STRIP TO A TEMPERATURE IN THE RANGE FROM 850*C. TO 1000*C. FOR FROM 1 TO 10 MINUTES, RAPIDLY QUENCHING THE STRIP TO PRODUCE A DUCTILE STRUCTURE, COLD ROLLING THE STRIP TO EFFECT A REDUCTION OF AT LEAST 50% TO A SHEET OF PREDETERMINED THICKNESS, FINAL ANNEALING THE SHEET AT A TEMPERATURE OF 850*C. FOR FROM ABOUT 10 MINUTES TO FOUR HOURS IN AN ATMOSPHERE OF DRY HYDROGEN HAVING A DEW POINT OF -45*C. OR LOWER, COOLING THE SHEET IN A MAGNETIC FIELD OF AT LEAST 1.5 OERSTEDS AT A RATE OF FROM 1*C. PER MINUTE TO 5*C. PER MINUTE TO A TEMPERATURE OF FROM 550*C. TO 750*C., AND THEN RAPIDLY COOLING THE SHEET, THE SHEET WHEN TESTED IN TOROIDAL CORE FROM BEING CHARACTERIZED BY A HYSTERESIS CURVE OF A HIGH DEGREE OF RECTANGULARITY AS INDICATED BY A BY/BM RATIO OF AT LEAST ABOUT 70%, AND A 400 C.P.S. CORE LOSS OF LESS THAN 25 WATTS PER POUND AT 16 TO 18 KILOGAUSS. 